Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft drives the generator rotor either directly (“directly driven wind turbine” or “direct drive wind turbine”) or through the use of a gearbox.
An important auxiliary system generally provided on wind turbines is a pitch system. Pitch systems are employed for adapting the position of a wind turbine blade to varying wind conditions by rotating the blade along its longitudinal axis. In this respect, it is known to rotate a wind turbine blade in such a way that it generates less lift (and drag) when the wind speed increases. This way, even though the wind speed increases, the torque transmitted by the rotor to the generator remains substantially the same. It is furthermore also known to rotate wind turbine blades towards their stall position (so as to reduce the lift on the blades) when the wind speed increases. These wind turbines are sometimes referred to as “active-stall” wind turbines. Pitching may furthermore also be used for rotation of the blade towards its vane position, when a turbine is temporarily stopped or taken out of operation for e.g. maintenance.
Many pitch systems comprise an electric motor in the hub which drives an actuating gear. Said actuating gear (pinion) meshes with an annular gear provided on the wind turbine blade to set the wind turbine blade into rotation. It is also possible however, that the annular gear is provided on the hub, whereas the electric motor and actuator are mounted on the blade. Yet other actuating mechanisms, such as e.g. involving hydraulic actuators, are also known.
It is further known to provide an individual pitch system (comprising e.g. a separate motor and separate control) for each wind turbine blade of a rotor. It is also known to provide a common pitch system wherein the pitch angle of the blades is the same for all blades on a rotor. Such a common pitch system may comprise a single motor or may comprise a plurality of motors, one for each blade.
A common control strategy of a pitch system in a variable speed wind turbine is to maintain the blade in a predefined “below rated pitch position” at wind speeds equal to or below nominal wind speed (for example approximately 4 m/s-15 m/s). Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions depends however on the complete design of the wind turbine. Above the nominal speed (for example from approximately 15 m/s-25 m/s), the blades are rotated to maintain the aerodynamic torque delivered by the rotor substantially constant. When the wind turbine is not operating, the blades may assume a vane position (e.g. at or around 90° pitch angle) to minimize the loads on the blades. During most of the wind turbine's life, a blade may however be in the below rated pitch position. The nominal wind speed, cut-in wind speed and cut-out wind speed may of course vary depending on the wind turbine design.
During operation of the wind turbine, forces may be acting on the blades that result in a constantly varying torque around the blade's longitudinal axis. These forces may include the aerodynamic torque around the longitudinal axis of the blade and also the weight of the blade which may exercise a torque around the blade's longitudinal axis, depending on the blade's position. Both these forces are non-constant, largely cyclical and tend to rotate the blade out of the position determined by the pitch control system.
When a pitch system involving gearing is used, the varying torque may result in flanks of the teeth of the actuating gear (pinion) and annular gear repeatedly touching each other. This repetitive contact may lead to fretting corrosion and premature wear. Since the below rated pitch position is the prevailing position for most wind turbines, the contact between the teeth and its consequences is concentrated on the same teeth.
Some solutions for these problems are known. It is e.g. known to provide an automatic lubrication system to try and prevent fretting corrosion. For example, DE 20 2005 014 699 U and EP 1 816 346 provide such lubrication systems. These lubrication systems may help to reduce fretting corrosion to a smaller or larger extent, but do not combat or resolve the problem underlying the corrosion. There still exists a need for a pitch system in a wind turbine which does not suffer from fretting corrosion. It is an object of the present invention to at least partially fulfil this need. Further advantages of the present invention will become apparent from its description.